The present invention relates to a spherical sealing body used for exhaust pipe joints in automobiles and a method of manufacturing thereof.
A spherical sealing body used for exhaust pipe joints is disclosed, for example, in Japanese Patent Application Laying Open No. 54-76759 (hereinafter referred to as "related art I"). The spherical sealing body disclosed in the related art I has heat resistance, excellent fitness with mating members and remarkably improved impact shock resistance but it involves, on the other hand, a drawback of often generating abnormal frictional noise during sliding movement under dry frictional conditions.
It is considered that the above-mentioned drawback in the sealing body is due to the great difference between the static friction coefficient and the kinetic friction coefficient of heat resistance material constituting the sealing body (expansive graphite, etc.) and also to that the frictional resistance of the sealing body made of such heat resistance material relative to the sliding velocity shows a negative resistance (that is, a phenomenon in which the friction coefficient is reduced in accordance with the increase in the sliding velocity).
In view of the above, the present applicant has already proposed a sliding (sealing) member which can overcome the foregoing drawback and suitably be used to exhaust pipe joints in Japanese Patent Application No. 56-120701 (Japanese Patent Laying Open No. 58-24620) (hereinafter referred to as "related art II").
That is, the sealing body in the related art II is prepared by molding refractory material as a mixture of one or more of expensive graphite, mica and asbesto together with reinforcing material comprising metal fibers, fine metal wires or metal gages obtained by weaving or knitting them, in which a lubricant composition comprising a polytetrafluoroethylene resin or a copolymer of tetrafluoroethylene and hexafluoropropylene is deposited to the surface of the sealing body.
In this sealing body, the lubricant composition deposited to the surface thereof can provide effects of reducing the friction coefficient, prevention of the refractory matrix material from transferring to mating members, reduction of the difference between the static friction coefficient and kinetic friction coefficient, etc., in addition, the tetrafluoroethylene resin does not show negative frictional resistance relative to the sliding velocity, consequently, there can be obtained an advantageous effect of suppressing the generation of self-excited vibrations due to "deposition-sliding", thereby contributing to the prevention for the generation of abnormal frictional noise in addition to the foregoing effect.
Now it has been found that although the sealing body based on the related art II described above can overcome the drawbacks in the related art I in view of the performance, there is a problem in this case for the method of manufacturing the sealing body.
The method of manufacturing the sealing body in the related art II is as described below.
(1) A method of coating adhesives to the surface of a sealing body substrate obtained by molding refractory material and reinforcing material together, and then scattering and depositing a tetrafluoroethylene resin powder thereover, or (2) a method of properly diluting a mixture of a tetrafluoroethylene resin powder and adhesives with a volatile solvent, brush-coating or spraying the solution to the surface of the sealing body substrate, or
(1) a method of coating adhesives to the surface of a sheet-like refractory material and then scattering and depositing a tetrafluoroethylene resin powder or (2) a method of properly diluting a mixture of a tetrafluoroethylene resin powder and adhesives with a volatile solvent and then brush-coating or spraying the solution to the surface of the sheet-like refractory material.
Any of the former methods of depositing the tetrafluoroethylene resin to the surface of the sealing body substrate requires an additional step of compressing the sealing body substrate again in a mold for adjusting the size and smoothing the surface after the deposition. In the latter methods, the method (1) requires a step of coating adhesives and the method (2) of applying brush coating brings about a difficulty in thin and uniform coating on the surface of the sheet-like refractory material. Accordingly, a method of depositing by means of spraying has been recommended in view of practical use.
However, although the method of depositing the solution by means of spraying to the surface of the sheet-like refractory material enables thin and uniform deposition on the surface, it has been found that a considerably great amount of the tetrafluoroethylene resin scatters around the sheet-like refractory material to cause loses in the tetrafluoroethylene resin, that is, poor material yield.